The invention relates to the field of fiber optic sensors, and more particularly, to a fiber optic sensor system in which a transducer modulates the frequency of an optical pulse circulating in a loop of optical fiber to convey sensor data.
Optical fiber technology is replacing electrically based transducer systems in many applications. Fiber optic sensor systems offer advantages such as geometric versatility, low-loss telemetry, high bandwidth, immunity to electromagnetic interference, and high sensitivity in comparison to conventional sensor systems interconnected by wire cables.
One class of fiber optic sensors is based on comparing the phase shift of light propagated through an optical fiber transducer by comparison with the phase of light propagated through a reference optical fiber. A phase shift results when the optical fiber transducer retards light propagation in response to the physical effect or condition which is being transduced. Some disadvantages of sensor systems which rely on phase shift measurement are that they require coherent light, and careful control of system geometry is required to measure the relative phase of the sensing light and the reference light.
Applications exist in which it is desirable to deploy dozens or hundreds of acoustic transducers in arrays that are towed through the ocean or deployed on the ocean floor. Examples of such applications include seismic exploration and ocean surveillance. U.S. Pat. No. 5,051,965, "Acousto-Optical Marine Sensor Array" describes a fiber optic sensor array system based on interferometric measurement of phase shifts occurring as light pulses are propagated successively through a series of fiber optic transducing coils. The system has the desirable feature of enabling signals to be monitored from a large number of acoustic transducers using only two fiber optic signal lines. This is accomplished by multiplexing the return signals from the separate transducers into a single optical fiber. The system requires careful control of the time delay between successive sensors and sufficient coherence of the laser light for measurement of relative phase between successive pulses. However, appropriate time delay is difficult to achieve because path lengths between the sensors must be carefully controlled. Further, laser light coherence restricts the laser sources which can be employed.
Therefore, there is a need for a fiber optic sensor system which can be deployed in a multiplexed array system and which does not rely on measurement of phase shifts of coherent light.
The present invention also relates to sensing arrangements, and it more particularly relates to force transfer sensors with a high frequency filter for use in fiber optic transducers or like devices.
Several fiber-optical acoustic sensors have been described in the recent past and are described below.
In one sensor, two single-mode optical fibers are arranged in the form of an interferometer in which a length of one of the fibers is subjected to a magnetic or acoustic pressure field an forms the sensing arm. The other fiber is shielded from the field and forms the reference arm. Then, by the photoelastic effect, a phase change is induced in the sensing fiber. Recombining the light from the sensing arm with that from the reference arm results in interference fringes which give a measure of the magnitude of the magnetic field or the magnitude of the acoustic wave. Because the two fiber arms are physically separate, differential environmental conditions face each and seriously affect the interferometer stability. The state of polarization (SOP) of the light emerging from each fiber arm must be correct or the two will not completely interfere.
U.S. Pat. No. 4,442,350 to Rashleigh describes a sensor for detecting the presence of an environmental field condition such as acceleration, temperature change, magnetic or acoustic fields. The field is sensed by interference between two mutually orthogonal polarized eigenmodes in a single monomode optical fiber which may be disposed either linearly or wound on a mandrel made of compliant material for sensing an acoustic field. Polarized light propagated through the optical fiber is detected at its outlet independent of environmentally induced low frequency variations whereby the sensor may be maintained at quadrature and maximum sensitivity.
U.S. Pat. No. 4,951,271 to Garrett et al. discloses an omnidirectional hydrophone having an elastic shell which has a circular cross-section so that the circumference of the shell about different axes changes differentially when the shell is subjected to pressure variations. The differences in circumference are measured by an optical fiber interferometer having one leg wound about the equatorial circumference of the shell and another leg wound about its meridional circumference. The shell may be oblate such that it narrows along one axis and widens along the other when the shell is subjected to a pressure change.
Other fiber optic sensors are described in the following patents, and relate to the general field of the present invention:
U.S. Pat. No. 4,534,222 to Finch et al. relates to a vibration sensor that includes two matched coils of fiber-optic material. When the sensor experiences vibration, a differential pressure is exerted on the two fiber coils. The differential pressure results in a variation in the relative optical path lengths between the two fibers so that light beams transmitted through the two fibers are differently delayed, the phase difference therebetween being a detectable indication of the vibration applied to the sensor.
U.S. Pat. No. 4,893,930 to Garrett et al. relates to a multiple axis, fiber optic interferometer seismic sensor that is enclosed within a case and supported by a plurality of cylindrical silicone rubber mandrels. Each mandrel is wound with a length of optical fiber which has a reflective end and a transmissive end. When the case is displaced, the supports change diameter in response to the relative motion between the seismic mass and the case. This change in diameter is translated to a change in length of the optical fiber, that is responsive to the displacing vibrations.
U.S. Pat. No. 4,950,883 to Glenn describes an arrangement for sensing changes in a monitored parameter. It includes an optical fiber which has at least one sensing fiber length having a sensing portion. Two periodic gratings of the same periodicity are situated in the fiber each at a different end of the sensing fiber length. Such gratings are reflective to a predominant portion of any light that propagates in the fiber and has a wavelength in a stopband range around twice the periodicity. When a particular broadband coherent light is launched into the fiber toward a first grating, the predominant portion of the sensing light is reflected from the first grating and the remainder of the sensing light passes into the sensing fiber length, where resonant buildup of light at certain wavelengths that are located within the stopband range and depend on the length of the sensing portion as influenced by changes in the monitored parameter takes place, and the gratings are rendered substantially transparent to the sending light at the plurality of wavelengths following the buildup. The effect of gratings and of the sensing length on the wavelengths of the light emerging from one of the end portions of the optical fiber is then detected.
U.S. Pat. No. 5,051,965 to Poorman relates to an acousto-optical sensor array which includes a distributed set of optical-fiber sensing coils. A light pulse is launched through the sensing coils in a serial order. The light pulse is cumulatively data modulated by the respective sensing coils and is returned as a time division multiplexed pulse train. The pulse train is split into a first pulse train and a retarded pulse train, such that the retardation time equals the travel-time delay of a light pulse between sensors. The phase shift between the retarded pulse train and the first pulse train is measured and indicates the quantity being sensed.
U.S. Pat. No. 5,056,884 to Quinlan, Jr. describes yet another fiber optical load sensing device, and U.S. Pat. No. 5,247,490 to Goepel et al. relates to a pressure compensated optical acoustic sensor.